Aspen Physical Property System - Physical Property Models

Like Huron and Vidal, the limiting case of infinite pressure is used. This
simplifies the expressions for Ai* and Am. Equation 2 becomes:
Where � depends on the equation-of-state (see Huron-Vidal Mixing Rules).
Equation 3 is completely analogous to the Huron-Vidal mixing rule for the
excess Gibbs energy at infinite pressure. (See equation 4, Huron-Vidal Mixing
Rules.) The excess Helmholtz energy can be approximated by the excess
Gibbs energy at low pressure from any liquid activity coefficient model. Using
the Helmholtz energy permits another mixing rule for b than the linear mixing
rule. The mixing rule for b is derived as follows. The second virial coefficient
must depend quadratically on the mole fraction:
With:
The relationship between the equation-of-state at low pressure and the virial
coefficient is:
Wong and Sandler discovered the following mixing rule to satisfy equation 4
(using equations 6 and 7):
The excess Helmholtz energy is almost independent of pressure. It can be
approximated by the Gibbs energy at low pressure. The difference between
the two functions is corrected by fitting kij until the excess Gibbs energy from
the equation-of-state (using the mixing rules 3 and 8) is equal to the excess
Gibbs energy computed by an activity coeffecient model. This is done at a
specific mole fraction and temperature.
This mixing rule accurately predicts the VLE of polar mixtures at high
pressures. UNIFAC or other activity coeffecient models and parameters from
the literature are used. Gas solubilities are not predicted. They must be
regressed from experimental data.
Unlike other (modified) Huron-Vidal mixing rules, the Wong and Sandler
mixing rule meets the theoretical limit at low pressure. The use of kij does
2 Thermodynamic Property Models 85
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